Add configuration to enable mail previews

Adds `config.mail_preview_enabled`

This allows mail previewing to be enabled easily in non-development
environments such as staging. The default is set to true for development
so no changes should be required to existing Rails applications.

I added this as a new config option in railties since it's required in
both actionmailer and in the railtie finisher. Otherwise we'd be placing
a dependency on `config.action_mailer.preview_path` in order to enable
mail previews.

The mail preview path can still be configured using the existing
`config.action_mailer.preview_path` configuration option.

Adding this prevents devs from having to do stuff like:

```ruby
  config.action_mailer.preview_path ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil
 
  routes.append do
    get '/rails/mailers'         => "rails/mailers#index"
    get '/rails/mailers/*path'   => "rails/mailers#preview"
  end
```

in order to enable mail previewing on staging.
As soon as I pushed this last night I realised that I wanted to redo this without adding this extra stuff here. In terms of migration path it seems easier to just default this to true for development environments and false for others.